1. Field of the Invention
This invention relates to a drilling machine used in drilling circuit component-mounting holes, throughholes and other small apertures in a printed circuit board.
2. Description of the Prior Art
In a printed circuit board drilling machine of a type known in the art, a plurality of printed circuit boards are placed on the top side of a table movable in a horizontal plane, and a plurality of drills are supported at locations above the table so as to be movable vertically. After the table has been moved to bring the circuit boards into position, the drills are lowered to drill holes in the boards.
In order to support printed circuit boards the number whereof corresponds to the number of drills, and in order to bear the thrust of these drills during the drilling operation, the table used in the conventional machine must have a large surface area and great rigidity, and is therefore of considerable weight. This not only increases the size of the overall apparatus but also necessitates a drive motor having a high rating in order to move the heavy table at high speed and position the table accurately.
In the drilling operation, moreover, frictional heat is produced due to friction caused by contraction of the hole immediately after drilling and the resulting pressing force acting upon the periphery of the drill, friction caused when cuttings travel upward in the drill groove, and friction produced between the cuttings in the drill groove and the inner wall of the hole. The frictional heat causes the fusing of smears on the inner wall surface of the hole, which in turn is an impediments to subsequent through-hole plating. In order to solve this problem, ultrasonic mechanical vibrations can be applied to the circuit board during the drilling operation to reduce frictional resistance and promote the discharge of cuttings from the drill groove. Experiments have shown that this procedure is effective to prevent smearing.
However, since the printed circuit boards are secured on the heavy table in the conventional apparatus, the ultrasonic vibration applied from the bottom side of the table cannot be transmitted to the printed circuit boards effectively due to damping and reflection.